ada-lang.c::advance_wild_match improve doc and parameter+temporaries types

This commit fixes the type of one of the parameters as well as a couple
of temporaries.

While at it, the function's description is slightly rewritten to make it
a little clearer what the function does.

gdb/ChangeLog:

        * ada-lang.c (advance_wild_match): Rewrite the function's
        description.  Change the type of target0, t0 and t1 to char.